Articular cartilage is hyaline cartridge that is composed of a small number of cells, collagenous extracellular matrix, abundant proteoglycans and water. Since vascular and neural networks are present in the bone and the bone has self-repairing ability, even when a bone is fractured, the fractured site is often adequately repaired. Articular cartilage, however, lacks vascular and neural networks. Accordingly, it has very little self-repairing ability and thus a cartilage defect lesion is not adequately repaired particularly when a large defect lesion is formed in the cartilage. Even when the lesion is repaired, the lesion is remodeled with fibrous cartilage that has different mechanical property from that of hyaline cartilage. Therefore, a cartilage defect lesion leads to pain in the joint and loss of joint function, which often result in development of osteoarthritis. In addition, an early stage of osteoarthritis, where abrasion of the surface of articular cartilage starts due to aging or excessive joint usage, may lead to cartilage defect over a large region as a result of progression of the symptoms.
Thus, since articular cartilage is inadequate in self-repairing ability, surgical procedures for treating cartilage lesion, for example, autologous osteochondral implantation (mosaicplasty), a procedure of perforating with a pick (microfracture), drilling, a procedure of shaving subchondral bone with a burr (abrasion procedure) and resection of injured cartilage (debridement) are required. Among these procedures, the microfracture procedure, drilling and the abrasion procedure are called marrow stimulation techniques, which stimulate bleeding from bone marrow to induce marrow-derived cartilage precursor cells in expectation of them to differentiate into cartilage. These techniques, however, have limitation for a widespread cartilage defect and cartilage regenerated by these procedures is fibrous cartilage that has different mechanical property from that of hyaline cartilage.
In 1984, Peterson et al. and Grande et al. tested autologous chondrocyte implantation (ACI) technique in the non-full thickness of rabbit articular cartilage. ACI is a technique involving harvesting and culturing tissue from patient's own normal cartilage, implanting the cultured cells suspended in a medium in the affected area and covering the cartilage defect lesion with a piece of periosteum to prevent leakage of the cells. ACI technique was first clinically applied in 1994 and now has been used for more than 15 years. Until now, several clinically successful examples have been reported. Recent clinical examinations, however, also report that ACI technique did not show significantly superior results over other surgeries with respect to repair of an articular cartilage defect.
There are two major reasons for such poor results from the ACI technique. One reason is the technical difficulties in anchoring cells and a scaffold to the joint defect lesion and covering them with a periosteum patch. In ACI technique, the joint needs to be widely exposed by arthrotomy to suture the periosteum patch to cover the cell suspension. Moreover, several complicated issues related to the periosteum patch including thickening, defect and intra-articular adhesion of periosteum have also been reported. The other reason is the limitation to the use of chondrocytes. Chondrocytes rapidly lose their differentiation phenotypes in monolayer culture and transform into fibroblasts. Another problem is that harvest of cartilage from a non-weight-bearing site of the affected joint required by the ACI technique leaves the problem of the donor site that has been harvested of chondrocytes to remain problematic.
Meanwhile, there has been an attempt to utilize natural polymers such as collagen, chitosan, agarose and alginic acid in regenerative medicine for articular cartilage. It is, however, difficult to acquire sufficient cartilage regeneration effect with polymer alone, and polymer is usually used in combination with cultured chondrocytes and mesenchymal stem cells. For example, Patent Document 1 discloses that mesenchymal stem cells embedded in a composition containing a monovalent metal salt of a low endotoxin alginic acid can be applied to a cartilage injury lesion to gain favorable hyaline cartilage regeneration that is almost comparable to normal cartilage. In addition, an attempt to use growth factors and cytokines has also been made in cartilage regeneration medicine. TGF-β and bFGF are known as typical factors for differentiating/proliferating chondrocytes.
SDF-1 (Stromal cell-derived factor 1) is one type of chemokines. SDF-1 is expressed in ischemic tissues caused by cardiac infarction, brain infarction, and skin lesion and the like to allow the vascular precursor cells to migrate toward the ischemic site for angiogenesis. Non-patent Document 1 describes that SDF-1 expression was confirmed in the bone implantation site and that SDF-1 played a role in guiding mesenchymal stem cells to the local site upon bone healing. Patent Document 2 discloses a sustained-release composition comprising SDF-1 and a hydrogel of modified gelatin. This composition is utilized for the treatment of ischemic diseases and the like because it allows sustained release of SDF-1. On the other hand, whether or not SDF-1 is expressed at the cartilage injury lesion and whether or not it exerts a beneficial effect upon administration to the cartilage injury lesion have previously been unknown. There is also a report saying that SDF-1 is present at a higher concentration in a pathological tissue of osteoarthritis or rheumatoid arthritis than in a normal tissue, and that SDF-1 of a higher concentration (200 ng/ml or higher, which is less than 100 ng/ml in a normal tissue) can result in necrosis of chondrocytes (Non-patent Document 2).
Patent Document 3 discloses a cell-free graft comprising an open porous structuring matrix and a serum. This cell-free graft is described of its potential to be used for cartilage regeneration since the serum in the graft can stimulate migration of mesenchymal precursor cells to the defect lesion, but whether or not it actually exerts a cartilage regeneration effect in vivo remains unrevealed. Generally, in cartilage regeneration medicine, it is difficult to predict the in vivo effect of implantation therapy only by in vitro cartilage regeneration tests or cell migration tests. Specifically, a graft used in cartilage regeneration medicine needs to provide performances, for example, to remain at the injury lesion for weeks, to successfully graft with the surrounding tissue as regenerated cartilage and the like. It is difficult to confirm such performances other than in vivo.
Patent Document 4 describes that a cell-free scaffold containing SDF-1 or TGF-β is arranged in a manner to allow fluid communication with cells, thereby rending the cells to migrate toward the scaffold. The scaffold actually used in Patent Document 4 is collagen sponge covered with cross-linked calcium alginate. Gelatin microspheres containing SDF-1 and/or TGF-β are embedded in the cross-linked calcium alginate layer of the scaffold. Patent Document 4 reports that when this scaffold was brought into contact with bone-marrow-derived mesenchymal stem cells (MSC), human adipose-derived stem cells (ASC) and synovial stem cells (SYN), with the collagen sponge side facing down, the cells in some cases had migrated to the collagen sponge of the scaffold. It also describes that although cartilage formation was confirmed with the scaffold containing TGF-β, the cell migration thereof was moderate, whereas the scaffold containing both TGF-β and SDF-1 resulted in good cell migration and cartilage formation were confirmed. Patent Document 4, however, does not show that the cells migrate to the cross-linked calcium alginate layer of the scaffold. Patent Document 4 also concludes that although MSC and ASC migrated to the scaffold containing only SDF-1 and not TGF-β, it was inadequate to induce cartilage formation with SDF-1 alone.